1. Field of the Invention
The present invention relates to pressure vessels and, more specifically, to apparatus and methods for reinforcing pressure vessels. Examples of such pressure vessels include rocket motors, gas generators, and the like.
1. Description of the Related Art
It is desirable in the design and construction of certain pressure vessels, for example, high performance pressure vessels for rocketry or gas generator applications, to effectively reduce or minimize the inert mass of the vessel while maintaining or enhancing its strength and reliability. It also has been desirable to do so while containing or reducing the cost of such vessels.
Many pressure vessel designs include one or more doubly curved surfaces, for example, such as those associated with integrally formed end domes. These doubly curved surfaces may assume a number of different specific geometries or geometric cross sections. Pressure vessels used as motor casings for rocket motors, for example, typically include a cylindrical section, a domed fore section or domed fore and aft sections, and a doubly curved transition or mating section disposed between the cylindrical section and the domed section or sections. Other vessel designs also include such doubly curved surfaces, for example, such as spheres, ellipsoids, and the like.
It is generally desirable in pressure vessel design to address weak points in the vessel structure so that the vessel has a predictable, consistent, repeatable failure mode and location. By predetermining this failure mode and location, the designer may ensure that the vessel is constructed such that the operating demand of the vessel will not exceed its strength at this weakest point. In a cylindrical vessel with end domes, for example, the desired failure location often preferably is in the cylindrical section. In more weight-optimized high performance vessels, the failure location may tend to move into the end dome region, for example, due to the stress peaking from bending.
In many applications, the doubly curved surface is the area, or an area, of the vessel that is most prone to mechanical failure. This can occur for a number of reasons. As a vessel, e.g., a rocket motor, is pressurized, it experiences a number of internal stresses. One such stress is attributable to the outward force exerted from the pressurized fluid in the interior of the vessel. Depending upon the application, another such stress may be the bending moments that are exerted, particularly at the doubly curved surface, associated with relatively opposed or noncooperating forces exerted on the components of the vessel. The stress peak may be especially severe at the junction between the doubly curved surface and a nondoubly curved surface, for example, such as the junction between a hemispherical end dome and a cylindrical wall. The vessel design ideally is such that it can accommodate these bending stresses at their peaks.
Pressure vessels may be made in a number of different ways. A typical manufacturing technique used to produce solid fuel rocket motors, for example, as described in U.S. Pat. No. 4,118,262, involves the use of helically wound fiber-based composites. A material made of a high strength, continuous reinforcing filament, such as graphite or aramid fiber, for example, is impregnated with a compatible resin for hardening. The filament is wound around a mandrel, the resin is set or hardened, and the mandrel is withdrawn from the casing. The fibrous material is stabilized on the mandrel by winding around pegs in the mandrel that define holes in the fibrous material. The angle between the axis of the fiber and the longitudinal axis of the casing typically is kept as small as possible to cause forces applied to the fibers to be substantially longitudinal tensile forces. The motor shell is subsequently filled with an insulating layer, a binding layer, and a solid fuel. The filament may be woven into strips or deposited as tow. According to U.S. Pat. No. 5,348,603 to Yorgasen, for example, similar manufacturing processes may be used to wind a motor casing that is inserted into a metal shell and expanded through use of a bladder in the mandrel prior to hardening of the resin. The casing adheres to the inside of the metal shell.
In view of the design approaches used in constructing pressure vessels and the tendency for mechanical failure associated with the doubly curved surface in high performance lightweight vessel applications, it is often desirable to strengthen this doubly curved surface. One approach to providing such added strength is to reinforce that portion of the vessel by applying a reinforcing structure or material at the exterior of the vessel. A reinforcing structure, for example, may be placed over the doubly curved surface and end dome. To provide a specific example, a filament-wound helical layer could be placed over the doubly curved surface, e.g., over the end dome and transition section of a cylindrical vessel with dome, to serve as a reinforcing structure. These reinforcing structures normally would be placed upon a pressure vessel after the initial vessel has been formed, e.g., by helical winding on the mandrel. The portion or portions of the reinforcing structure that are not required for vessel reinforcement then can be removed to limit the added weight penalties created by this approach.
This approach is generally limited or disadvantageous, for example, in that it tends to be labor intensive. It also can add unnecessarily to the cost of the reinforced vessel, for example, in that the reinforcing material, which often involves considerable expense, typically is discarded and cannot readily be reused. This approach also can be limited or disadvantageous, for example, in that, as noted above, the weight of the vessel often is increased unnecessarily, especially where filament winding is used as the reinforcing material. Filament winding also can result in increased thickness at or around the polar openings of the vessel, where present, for cylindrical vessels.
Accordingly, the present invention provides a pressure vessel and related apparatus and methods wherein the pressure vessel can withstand relatively high stresses relative to its mass.
The present invention also provides a pressure vessel and related apparatus and methods that achieve the foregoing with relative cost effectiveness.
A pressure vessel and associated methodology are provided to achieve the foregoing in accordance with the purposes of the invention as embodied and broadly described in this document.
The vessel includes a vessel body having a doubly curved surface disposed about a vessel body axis. A reinforcing structure is disposed at the doubly curved surface of the vessel body about the vessel body axis. The reinforcing structure comprises a plurality of gore pieces disposed on the vessel body at the doubly curved surface to form a gore body. Each of the gore pieces comprises a composite sheet having a shape that conforms to the doubly curved surface. The gore pieces are disposed on the doubly curved surface in overlapping relation.
Each of the gore pieces preferably comprises a fiber-resin preimpregnated (also referred to as xe2x80x9cprepregxe2x80x9d) composite material. The resin serves as a bonding material that bonds the gore pieces to one another. Each of the gore pieces preferably has a longitudinal dimension and comprises fibers oriented in a plus/minus configuration relative to the longitudinal dimension. The fibers in the plus/minus configuration of each of the gore pieces are preferably oriented at a fiber angle of between about 30xc2x0 and 60xc2x0, and more preferably about 45xc2x0, with respect to the gore piece longitudinal axis. The gore pieces are any sheet or partial sheet of material that tapers in a direction, such as a triangular, truncated triangular or rhombic shape, and these are preferably assembled in overlapping relationship to one another as a single layer in a flattened xe2x80x9cZxe2x80x9d configuration or multiple layers where the gore pieces of each layer abut at their meridional edges. Where there are multiple layers, additional fiber or tow may be wound to separate each layer or, alternatively, the layers are adjacent and contact one another. In any manner of placement, it is preferred that the gore pieces overlap one another by about 40 to 60%, and most preferably by about 50%, to provide the gore body with a substantially uniform thickness.
The vessel has an axis of symmetry and each of the gore pieces also has an axis of symmetry. The longitudinal axis of each gore piece curves, but an individual gore piece may be rotated for viewing as a straight line. From this perspective, in some embodiments, it is preferred to have the gore piece longitudinal axis aligned with the axis of symmetry of the vessel. In other embodiments, especially where the gore pieces may be placed by machine, it is expedient, due to machine winding techniques, to preferably offset the gore piece longitudinal axis with respect to the vessel axis of symmetry. In this latter case, the offset angle is preferably less than or equal to about plus or minus 20xc2x0 and is more preferably between plus or minus 15xc2x0. A more preferred orientation of the gore piece deviates from the axis of symmetry by no more than 10xc2x0, a deviation of 5xc2x0 or less is most preferred, and a deviation of 0xc2x0 is most preferred.
In another embodiment, the vessel-reinforcing structure may be assembled independently of the pressure vessel itself for later assembly and coupling with the vessel. In the intended environment of use, the pressure vessel has a doubly curved surface and presents a vessel body axis about which the doubly curved surface may be formed. As described above, the vessel-reinforcing structure comprises a plurality of gore pieces that are shaped to conform to the doubly curved surface of the vessel body about the vessel body longitudinal axis and to form a gore body. Each of the gore pieces comprises a composite sheet and is bound with resin in the assembled vessel-reinforcing structure.
Another embodiment relates to a method for making a reinforcing structure for a vessel having a vessel body with a doubly curved surface. The method steps entail making a plurality of gore pieces for disposition on the vessel body at the doubly curved surface about the vessel body axis to form a gore body. The gore pieces are each disposed as a composite sheet. The gore pieces each have a longitudinal dimension and comprise fibers oriented in a plus/minus configuration relative to the gore piece longitudinal dimension.
Preferred aspects of the method include orienting the fibers in the plus/minus configuration of each of the gore pieces at a fiber angle of between about 30xc2x0 and 60xc2x0 with respect to the gore piece longitudinal axis, and the most preferred orientation is 45xc2x0. The gore pieces are preferably formed in a substantially triangular or rhombic shape. A bonding material, such as a synthetic resin, is placed on each of the gore pieces, which are assembled in overlapping relation to one another.
The gore pieces can be assembled by a variety of techniques to place them in overlapping relation to one another. According to one method of disposition, which is most preferred, first layer gore pieces substantially abut one another at their respective meridional edges around the doubly curved surface, and second layer gore pieces are disposed over the first layer gore pieces and around the doubly curved surface so that each of the second layer gore pieces overlaps at least two of the first layer gore pieces by about 40 to 60%. The most preferred extent of overlap is about 50%. An alternative method of overlapping the gore pieces is to assemble them as a single layer where the adjacent pieces overlap one another in a flattened xe2x80x9cZxe2x80x9d configuration. The extent of overlap between adjacent pieces is preferably from 40 to 60%, and is more preferably about 50%. Either method of disposition preferably results in the gore body having a substantially uniform thickness.
Another aspect of this method of making the vessel-reinforcing structure is to assemble the vessel-reinforcing structure apart from the pressure vessel, as opposed to assembly on the vessel. The gore body may be assembled using the gore pieces and resin, and the assembly may be placed on a pressure vessel prior to hardening of the resin. Alternatively, the resin may be allowed to harden and may be adhered to the pressure vessel using additional resin at the time of assembly.